I have invented and tested a device to allow the top-down build of domes approximately 16' high. 16′ tall domes fall into a gray area: too big to conveniently build with ladders, to small to be worth renting a crane. That’s where this hoist comes in. If you’ve got a 16′ dome or thereabouts and you’re sick of wrangling struts while perched precariously on a ladder 15′ in the air, this is for you.

The advantages of top-down construction are, first, that you can do all the work on the ground. This is obviously safer than doing the work on a ladder. It’s also faster, since you don’t spend time climbing up and down, and moving, a ladder. It’s also easier on your legs. The main advantage, however, is that you can have multiple people help with the build, which means that the more helpers you have, the faster the build goes. This is not possible with a bottom-up build, where you can only do as much work as you have ladders, and given how much tall ladders cost, you probably don’t have very many ladders.

The parts used to build this device are readily available and cost me about $300.

If eplaya had a "like" button like facebook, I'd press it. I've never made a dome, but if I were to make a big one, I'd seriously look into making a dome hoist like yours. I like your dome links too. Thanks.

I considered scaffolding, but rejected it. You can get a set of 6' high stackable scaffolding for about $170. Two of those would be in the same ballpark as my mast and get you to a height of 12', from which an average-height person would be able to reach the top of the dome. But I can already accomplish the same thing using a 14' ladder, and that only cost me $150. Scaffolding has a wider work surface than a ladder, which is good, but it doesn't address any of the other reasons I built the mast. You can still only work on one vertex at a time, which prevents parallelism and limits others' ability to help the build. You still have to move the scaffolding around the dome as you work, which slows the build down. And you still have to climb up and down the scaffolding as you work, which slows the build down and restricts who can help (since not everyone is physically capable of that level of exertion, or may not be capable of climbing a ladder at all). Finally, you are still working above the ground, with the associated fall risk, not to mention the annoyance of dropped tools, bolts, washers, and so forth.

If I wanted to do a bottom-up build, I would just use ladders, which is what I've done in the past. The point of the hoist not to find a novel way to build the dome. I know how to build the dome. The point is to make the advantages of a top-down build accessible to people who don't have a crane.

BBadger wrote:A guylined pole with a pulley attached. Useful, but I don't know if it is really an invention...

The reason I call it an "invention" is because a lot of ingenuity and creativity went into making it out of readily-accessible and cheap parts. If I wanted to spend thousands of dollars, I would just have gone to a welder and said, "make me this," and then I would agree with you. The reason I call it an invention is because I searched and searched to try to find someone who had solved this problem (doing a top-down build without a crane) before me, and I didn't find anything, so I conclude I'm the first to build it.

Bob wrote:I'm uncomfortable with plumbing threads, and I think clove hitches tied off with two half-hitches would secure the guy lines better than cow hitches, but if it works it works.

Re: pulleys, you can find them at tree trimmer supply outlets and hardware stores that carry such things. Jackson's in San Rafael CA should have them. Maybe REI as well.

I did some weight-testing of the mast that isn't documented in the blog post. Basically, I supported the ends and put weight on the middle to see how much weight it could take for how much deflection. With 100 lbs of weight in the middle, it flexed and then returned to true. With about 200 lbs of weight on the middle, the mast did not return fully to true after this amount of deflection, which indicates some permanent deformation of the metal. (For the record, I flipped it 180 degrees and bent it back.) I conclude that 200 lbs is too much weight, but 100 is "okay".

This test gives a perspective on the strength of the metal and the threads, but it does not directly translate to the real world, because in the real world, the ends are fixed and the load is basically at one end point. If you think about it, if the mast was perfectly vertical and the load was perfectly balanced over the center of the mast, it could support a huge amount of weight, because all force would be straight down through the pipe, and you would have to literally crush the wall of the pipe to cause a failure. So, when we're talking loads of 700 lbs or so, it's only forces perpendicular to the mast that matter, not forces parallel to the mast. Because we keep the mast close to vertical, perpendicular forces are minimized, although they can't be eliminated completely because the pulley is off to one side of the mast. The pipe in use is 2" diameter, which translates to about 0.25 degree off-axis over its 25-foot length, or about 0.3% of the dome's weight applied as perpendicular force to the mast when the mast is perfectly upright.

Also, the weight is being applied to the mast very close to the top guy point. The worst-case scenario is that you guy the end and apply weight in the middle, or that you guy the middle and put weight at the end, both of which provide maximum leverage to the weight. The best-case scenario is that you guy the pole right where the weight is being applied, which minimizes leverage and transfers the forces directly to the guy lines. That's the scenario that's in play here.

Honestly, I think that you will be more likely to bend off the pole than you would be to rip out the threads. If I had $50 to spare on the experiment, I would put weight on a mast until it bent off and see where it failed, but I don't care that much.

Bear in mind that the cow-hitch is formed from a loop made of a figure-eight knot, so both ends of the rope are captured. There is no danger of the loose end of the rope working its way out of the hitch. If the line goes slack, there is a theoretical possibility of the cow hitch working its way loose, but I have never had this happen, and of course once the lines are loaded, the hitches pull tight. You're right, though, that clove hitches tied with half-hitches would be more secure.

I wasn't able to find anything sold as a "pulley" that would take the weight of this project. I searched around forever trying to find the right search term before I found "snatch block." One common use for snatch blocks is directing falling trees, so I agree that landscaping supply stores might be a good choice. They're also used for recovery of stuck off-road vehicles, if that helps. Of course, there's always the Internet.

Only situation where I've seen similar rigs is for backyard boat building/repair -- they call them "gin poles". Bet you could also use an A-frame arrangement if you wanted to fiddle with welding.

Something simpler and much lighter might use Sched 40 or better aluminum pipe in a full 20 ft length. I've used salvaged aluminum 1-1/2" pipe for shade frames. Threaded steel certainly is more obtainable, but the threaded joint in the middle would worry me wrt bending.

Bob wrote:Only situation where I've seen similar rigs is for backyard boat building/repair -- they call them "gin poles". Bet you could also use an A-frame arrangement if you wanted to fiddle with welding.

Something simpler and much lighter might use Sched 40 or better aluminum pipe in a full 20 ft length. I've used salvaged aluminum 1-1/2" pipe for shade frames. Threaded steel certainly is more obtainable, but the threaded joint in the middle would worry me wrt bending.

The 20-foot pole would definitely be stronger, but it would also be nigh-impossible to transport, at least with my rig (I have a 16-foot trailer that I use to bring my camp to burns). The advantage of the threaded approach is that it breaks down to 10-foot lengths.

My original idea was to build three-pointed pyramid. One problem with that is that the A-frame would have to be bigger than the dome itself, to allow the dome to be lifted up inside it. This would have required legs of about 35 feet, which is way beyond my ability to fabricate, and additional bulk to transport. If the legs of the frame are inside the dome, then they get in the way as the dome is raised up, although I can imagine a scenario where the poles fit down through the struts of the "top" intersection and didn't get in the way as it was raised.

Regarding bending the threaded joint, I agree that is the weak spot in the entire contraption, however, by keeping the mast vertical and keeping the load parallel to the mast, that weakness is removed from the equation. Also, keep in mind that for much of the dome's construction, the weight is much less than the maximum. It's only the last few layers where things start to get really heavy. Also, it became apparent from the test-build that if the mast did fail, it would probably be non-catastrophic. For most of the build, some part of the dome is resting on the ground and some other part of the dome is up in the air being worked on. In the event of a failure, you would probably damage your dome, but unless the mast itself hit somebody on the head, there probably wouldn't be any serious injuries. Even if the mast did bend, it would still be guyed at the top and couldn't fall over to hurt someone. I think that the most likely failure mode is a slow-motion bend-off of the mast, which I believe would happen relatively gracefully.

BBadger wrote:A guylined pole with a pulley attached. Useful, but I don't know if it is really an invention...

He didn't invent the pulley, he invented a new and innovative way of using them for a specific task. I think that would qualify as an invention.

Not a pulley. A "guylined pole with a pulley attached." It's essentially a fixed gin pole.

I'm not knocking him for his effort or the guide. It's good and useful information, but it's not an invention. It's not a new way to do things. It's an improved bill of materials for constructing the gin pole.

"The essence of tyranny is not iron law. It is capricious law." -- Christopher Hitchens

BBadger wrote:A guylined pole with a pulley attached. Useful, but I don't know if it is really an invention...

He didn't invent the pulley, he invented a new and innovative way of using them for a specific task. I think that would qualify as an invention.

Not a pulley. A "guylined pole with a pulley attached." It's essentially a fixed gin pole.

I'm not knocking him for his effort or the guide. It's good and useful information, but it's not an invention. It's not a new way to do things. It's an improved bill of materials for constructing the gin pole.

I'll remove the reference to the word "invented" if we can focus on the device itself and not the semantics.